Most motor vehicles today include disc brake systems for the front axle wheel assemblies and many further include disc brakes at the rear axle position. The disc brake rotor is a circular metal disc having opposed braking surfaces that are clamped by brake pads carried by a brake caliper to exert a braking effect. The wheel hub typically incorporates an anti-friction wheel bearing assembly in which one race of the bearing is coupled to the vehicle suspension and the other rotationally mounts the wheel hub, the brake rotor and wheel. Ordinarily, the rotating components of the rotor and hub assembly are manufactured separately and assembled together. This enables the brake rotor to be serviced and replaced if necessary during use. Moreover, the desired material characteristics for a brake rotor and the hub components are different. Although efforts to integrate these components have been proposed, such an approach has not found widespread acceptance.
In order to enhance performance of the braking system, it is desired to carefully and accurately control the dimensional characteristics of the rotor braking surfaces as the rotor rotates. The thickness variation of the disc and the lateral run-out or lateral deflection of the surfaces as they rotate need to be held to minimum tolerances. However, manufacturers have faced difficulties in achieving enhanced control over these tolerances due to the influence of several factors.
Most efforts to date have focused on decreasing run-out by controlling the dimensional characteristics of the rotor and therefore the relationship of the rotor surface to the wheel hub flange or surface. However, despite the fact that the tolerances and dimensional characteristics of the rotors have improved, performance and run-out problems still exist. These run-out problems are due in large part to other components of the wheel end assembly, including the bearing/hub assembly, which is comprised of a wheel hub and a bearing or the knuckle/hub assembly, which is comprised of a knuckle, a wheel hub, and a bearing.
One factor that contributes to this run-out is the stack-up of the individual components in a rotor/knuckle/hub assembly, i.e., their combined tolerances. While the tolerances of each part can be reduced when they are separately machined, when the parts are assembled, the combined tolerances stack up, causing run-out that is still relatively significant. Another factor that contributes to stack-up is any variation in the turning processes that are used to machine the wheel hub flange surface or the rotor surface, when the wheel hub and the rotor are individually machined, in an effort to make them flat. Further, the installation and press condition of the wheel bolts, the assembly process of the rotor/knuckle/hub assembly, and improperly pre-loaded bearings, can all cause misalignment of the rotor surface with respect to the brake pads and thus cause unacceptable run-out. This run-out can cause premature failure of the brake lining due to uneven wear which requires premature replacement of the brake lining at an increased expense. Further, problems due to run-out include, brake judder, steering wheel “nibble” and pedal pulses felt by the user, and warped rotors which result in brake noise and uneven stopping.
Presently available manufacturing methods and designs of rotors and knuckle hub assemblies to which the rotors are attached limit the accuracy to which lateral run-out of braking surfaces can be controlled. These methods and designs are also insufficient to solve the problems associated with run-out, as discussed above. Current methods typically involve finishing the rotor and the hub individually and then assembling the machined parts to form a completed brake rotor assembly. These methods, however, do not solve the run-out problems due to the factors discussed above, including stack-up tolerances, turning process variations, and wheel bolt and bearing installations.
Other options have been considered in an effort to solve the run-out problem, but they also all suffer from a variety of disadvantages. One contemplated option for reducing run-out is to separately decrease the run-out of each individual component, by decreasing their respective tolerances during manufacture and then assembling the components. The “stack up” of tolerance variations related to such an approach is still significant and provides only limited system improvement at an increased manufacturing cost. Another contemplated option includes tightening the press-fit tolerance variation between the knuckle, the wheel hub, and the bearing. This, however, significantly increases the difficulty in the assembly process as well as increases the manufacturing cost. Further, this option does not provide the desired reduction in system run-out.
It would therefore be advantageous to design a brake assembly for a motor vehicle that decreases system run-out without significantly increasing the manufacturing cost of the assembly or increasing the manufacturing difficulty.